Aluminization has been used for many years for protecting hot metal parts, and in particular for improving resistance to corrosion and to oxidation of the exposed surfaces of gas turbine blades. Aluminization consists in enriching with aluminum the metal of the part to be treated, the metal generally being an alloy, and the aluminum enrichment taking place in the vicinity of its surface. Aluminization gives the metal two sets of physical characteristics at high temperatures: the aluminum-rich outer layer guarantees satisfactory resistance to hot corrosion and oxidation without altering the mechanical properties of the non-treated parent metal.
The blades in the hot stages of modern turbines and aviation jet engines are cooled by injecting compressed air into cavities whose shapes are sometimes complex and which constitute cooling channels. Depending on the composition of the blades and the operating temperatures thereof, it is sometimes also necessary to provide protection for the inside surfaces created by those channels.
Various thermochemical methods are known for performing aluminization treatment.
In those methods, aluminium is added by adding or creating in situ a halogenated carrier which decomposes at the surface to be treated so as to add aluminum atoms thereto. The high temperature of such treatment causes a limited amount of intermolecular diffusion of the aluminum in the metal of the substrate constituted by the part to be treated. In this way, aluminum-rich layers are formed at said surface.
The aluminum-adding power of the halogenated carrier is a function of its partial pressure in the gaseous mixture which results from thermochemical equilibrium.
Aluminization treatment may be performed either in a "pack" or by vapor deposition.
Pack treatment is described firstly below.
A pack is constituted by a powder mixture:
an aluminum-rich metal alloy in powder form; PA1 an inert diluent such as alumina in powder form; and PA1 a halogenated carrier generator, such as ammonium chloride ClNH.sub.4.
The parts to be treated are disposed in the pack, inside a metal case which is then heated in a hydrogen atmosphere.
The halogenated carrier is formed and is charged with aluminum by contact with the particles of the pack. The carrier transports the aluminum by gaseous diffusion to the surface to be treated where it decomposes to give a gaseous decomposition residue. The residue comes back into contact with the metallic aluminum of the pack, thereby regenerating the halogenated carrier.
The equilibrium between the diffusion of the aluminum into the substrate and the gaseous diffusion, as a function of the density of the pack and of its richness in aluminum, has an influence on the aluminum content in the enriched layer which is formed in the substrate. As the pack becomes poorer in aluminum, the activity of the halogenated carrier decreases in the vicinity of the pack.
Vapor deposition treatment enables the activity of a halogenated carrier to be kept constant because it is enriched with aluminum independently of the position of the part. In this way, the metallurgical characteristics of the layer formed can be controlled more accurately.
However, the shapes of the part to be treated can pose problems. That is why it is often necessary to create an artificial flow inside the treatment enclosure. Given the treatment temperatures that are commonly used, it is sometimes difficult to control the quality of the flow easily and effectively.